VHF radar echoes observed in the summer and winter polar mesosphere over Andøya, Norway

Abstract

The mobile SOUSY VHF (53.5 MHz) Radar has been operated on a campaign basis on the island of Andøya in Northern Norway (69°17′N, 16°01′E), with the aim of investigating the backseattering structures and dynamics of the polar middle atmosphere, since November 1983. During winter the occurrence of mesospheric echoes at altitudes from 50 to 90 km is strongly correlated with radio wave absorption events measured with a riometer on 32.5 MHz. Individual structures are observed for up to 8 hours and, in general, appear as thin layers separated by 3–5 km, with typical maximum signal‐to‐noise ratios (SNRs) of 5–25 dB. Many of these are related to variations in the background wind produced by long‐period gravity waves and tidal period motions and, in as much, are similar to mesospheric layers at middle and low latitudes. In summer the echo region is largely restricted to a height interval from 75 to 95 km, with 80% of the echoes occurring in the 80‐ to 93‐km height range, and on average, it has a distinct maximum in SNR near 86 km. In the 83‐ to 91‐km height region, echoes are detected almost continuously, with a minimum occurrence rate of 85%. There is a low correlation (∼0.26) between echo strength and absorption. Many of the weaker echoing regions, those with SNRs up to 30 dB, are related to the background wind shear. Spectral analysis of time series of both SNR and zonal velocity for a period of 9 days reveals 54‐, 24‐, 12‐, and 8‐hour components, with the 12‐hour component dominating. Strong bursts in backscattered power tend to occur in the late afternoon and early morning hours, so they coincide with the time of maximum westward velocity of the semidiurnal tide. This suggests that the bursts are due to an increase in the turbulent intensity produced by dynamical instability of this mode. Layers consistent with dynamically unstable upward propagating inertio‐gravity waves are also often observed to move down through the 93‐ to 80‐km height range. Thus dynamical instability of the semidiurnal tide and long‐period gravity waves appears to play a major role in determining much of the structure observed in the backscatter region. However, the strongest summer polar mesopause echoes, those with peak SNRs greater than 30 dB, while showing modulation due to the background winds, often show no clear relation to the background wind shear. This supports the view that the production mechanism of the very strong SNRs associated with this layer is different in character from that of winter echoes observed in the polar mesosphere by VHF radars, and to those mesospheric layers observed at mid‐latitudes using the same technique.